Impacts of Cyclonic Eddy on Phytoplankton Biomass and Spring Bloom Onset Time in the Kuroshio Extension Region

Satellite chlorophyll-a concentration (CHL) data from 1997 to 2017 were analyzed to comprehend the impact of cyclonic eddy on phytoplankton biomass and spring bloom onset time in the Kuroshio Extension region (KE) of the western North Pacific Ocean. Prior to defining the spring bloom onset time, spatiotemporal gaps in satellite CHL data were filled by applying empirical orthogonal function-based interpolation. In general, based on datasets from all seasons, concurrences of negative sea surface height anomaly (SSHa) and positive CHL anomaly (CHLa) were observed, albeit not always. This indicates an eddy-induced upwelling supplies nutrients to the ocean surface layer that would promote phytoplankton growth. In this study, a Cumulative Sum method was applied to find phytoplankton spring bloom onset. Correlation between SSHa and spring bloom onset was investigated. We found that approximately over the active cyclonic eddy area, positive correlation between winter/late winter SSHa and spring bloom onset was observed. This positive correlation indicates that cyclonic eddies during the winter likely tended to advance phytoplankton spring bloom onset. Based on winter SSHa, cyclonic eddies were dominant within the periods of 1998~2001 and 2005~2009 (hereafter KE unstable period), whereas anti-cyclonic eddies were dominant within the periods of 2002~2004 and 2011~2015 (hereafter KE stable period). CHL during the KE unstable period was higher than that during the KE stable period. Furthermore, phytoplankton spring bloom onset also took place earlier during unstable period than during stable period. Perhaps, during unstable period, phytoplankton likely benefited from shallowed mixed layer (i.e., increased mean light availability within mixed layer) during the winter. This improved light condition was likely the factor behind the high winter CHL and earlier spring bloom onset during unstable KE period.